1. Field of the Invention
The present invention relates to an optical head for recording or reproducing information on or from a plurality of types of information recording media having different light transmitting layer thicknesses, an optical disc device including such an optical head and an information processing device including such an optical disc device.
2. Description of the Background Art
With the practical application of blue-violet semiconductor lasers, Blu-ray discs (Hereinafter, referred to as BDs), which are high-density and high-capacity optical information recording media having the same size as CDs (Compact Discs) and DVDs (Digital Versatile Discs), have been put to practical use.
A CD is an optical disc whose light transmitting layer has a thickness of 1.2 mm, for which laser light having a wavelength of about 785 nm and an objective lens having a numerical aperture (hereinafter, also referred to as an NA) of 0.45 to 0.52 are used to record or reproduce information and which has a storage capacity of about 650 Mbyte.
A DVD is an optical disc whose light transmitting layer has a thickness of about 0.6 mm, for which laser light having a wavelength of about 660 nm and an objective lens having an NA of 0.60 to 0.66 are used to record or reproduce information and which has a storage capacity per layer of about 4.7 Gbyte. There are two types of DVDs for practical use, i.e. single layer discs having one information recording surface and two-layer discs having two information recording surfaces.
On the other hand, a BD is an optical disc having information recorded or reproduced on or from an information recording surface having a light transmitting layer whose thickness is about 0.1 mm using a blue-violet laser light source for emitting blue-violet light having a wavelength of about 405 nm and an objective lens having an NA of about 0.85. There are also two types of BDs for practical use, i.e. single layer discs having one information recording surface and two-layer discs having two information recording surfaces, and a storage capacity per layer is about 25 Gbyte.
In the case of recording or reproducing information on or from a plurality of information recording surfaces as in the case of a BD, the thickness of the light transmitting layer differs for each information recording surface, wherefore a third-order spherical aberration is created on the information recording surface deviated from an optimal light transmitting layer thickness (thickness of the light transmitting layer at which a third-order spherical aberration is minimum when parallel light is incident on the objective lens) of an objective lens according to a deviation from the optimal light transmitting layer thickness. In the BD, if the light transmitting layer thickness deviation from the optimal light transmitting layer thickness is 10 μm, a third-order spherical aberration of about 100 mλ is created. Thus, an optical head for recording or reproducing information on or from the BD generally includes means for correcting the third-order spherical aberration.
For example, Japanese Unexamined Patent Publication No. H11-259906 discloses an optical disc device in which a collimator lens is mounted on a collimator lens actuator to move the collimator lens arranged between a light source and an objective lens in an optical axis direction and change a divergence angle or convergence angle of laser light incident on the objective lens so as to cancel out a third-order spherical aberration resulting from a light transmitting layer thickness deviation.
On the other hand, many of optical heads for high-density optical discs such as BDs each using short-wavelength laser light and an objective lens having a high NA include means for correcting a third-order coma aberration caused by the tilt of the optical disc (hereinafter, also referred to as “disc tilt”). In such an optical disc, a method for inclining the objective lens mounted on an objective lens actuator in a radial direction of the optical disc or a method using a liquid crystal element is, for example, put to practical use.
In recent years, there have been proposed compatible optical heads for recording or reproducing information on or from high-density optical discs such as CDs, DVDs and BDs by focusing laser light having three different wavelengths using a plurality of objective lenses.
Here, one construction of a conventional optical head is described with reference to FIG. 20. FIG. 20 is a diagram showing a schematic construction of the conventional optical head. In FIG. 20, an optical head 140 is provided with a blue-violet laser light source 101 for emitting blue-violet laser light, a relay lens 102, a polarization beam splitter 103, a collimator lens 104, a flat-plate mirror 105, a quarter-wave plate 106, a diffractive lens 107, an objective lens 108, an objective lens actuator 109, a two-wavelength laser light source 111 for emitting red laser light and infrared laser light, a diffraction grating 112, a flat-plate beam splitter 113, a collimator lens actuator 114, a wedge-shaped mirror 115, a quarter-wave plate 116, a compatible objective lens 118, a detection hologram 121, a detection lens 122, a light receiving element 123 and a front monitor sensor 124.
First of all, the operation of the optical head 140 in the case of recording or reproducing information on or from a BD 90 is described. The BD 90 includes two information recording surfaces L0, L1. Blue-violet laser light having a wavelength of about 405 nm and emitted from the blue-violet laser light source 101 is converted into divergent light having a different NA by the relay lens 102 and incident as S-polarized light on the polarization beam splitter 103. The blue-violet laser light reflected by the polarization beam splitter 103 is converted into substantially parallel light by the collimator lens 104 and passes through the wedge-shaped mirror 115 to be incident on the flat-plate mirror 105. A part of the blue-violet laser light incident on the flat-plate mirror 115 is reflected toward the quarter-wave plate 106. The other part of the blue-violet laser light incident on the flat-plate mirror 105 is incident on the front monitor sensor 124 after passing through the flat-plate mirror 105. Then, the output of the blue-violet laser light source 101 is controlled based on the output of the front monitor sensor 124.
On the other hand, the blue-violet laser light reflected by the flat-plate mirror 105 passes through the diffractive lens 107 after being converted into circularly polarized light by the quarter-wave plate 106. The blue-violet laser light having passed through the diffractive lens 107 is focused to be incident as a light spot on either one of the information recording surfaces L0 and L1 of the BD 90 by the objective lens 108.
The blue-violet laser light reflected by the specified information recording surface of the BD 90 is reflected by the flat-plate mirror 105 after passing through the objective lens 108 and the diffractive lens 107 again and being converted into linearly polarized light different from the one on an outward path by the quarter-wave plate 106. The blue-violet laser light reflected by the flat-plate mirror 105 is incident as P-polarized light on the polarization beam splitter 103 after passing through the wedge-shaped mirror 115 and the collimator lens 104. The blue-violet laser light having passed through the polarization beam splitter 103 is introduced to the light receiving element 123 via the flat-plate beam splitter 113, the detection hologram 121 and the detection lens 122. The blue-violet laser light detected by the light receiving element 123 is photoelectrically converted. A signal generated by the photoelectric conversion is calculated by an unillustrated controller to generate a focus error signal used to follow a surface runout of the BD 90 and a tracking error signal used to follow the eccentricity of the BD 90.
Next, the operation of the optical head 140 in the case of recording or reproducing information on or from a DVD 70 is described. Red laser light having a wavelength of about 660 nm and emitted from the two-wavelength laser light source 111 is separated into a main beam as 0th-order light and sub beams as ±1st-order diffracted light by the diffraction grating 112. The main beam and the sub beams are incident as S-polarized light on the flat-plate beam splitter 113. The red laser light reflected by the flat-plate beam splitter 113 passes through the polarization beam splitter 103 and is converted into substantially parallel light by the collimator lens 104 to be incident on the wedge-shaped mirror 115. A part of the red laser light incident on the wedge-shaped mirror 115 is reflected toward the quarter-wave plate 116. The other part of the red laser light incident on the wedge-shaped mirror 115 is incident on the front monitor sensor 124 after passing through the wedge-shaped mirror 115 and the flat-plate mirror 105. The output of the red laser light of the two-wavelength laser light source 111 is controlled based on the output of the front monitor sensor 124.
On the other hand, the red laser light reflected by the wedge-shaped mirror 115 is focused as a light spot on an information recording surface of the DVD 70 by the compatible objective lens 118 after being converted into circularly polarized light by the quarter-wave plate 116.
The red laser light reflected by the information recording surface of the DVD 70 is reflected by the wedge-shaped mirror 115 after passing through the compatible objective lens 118 again and being converted into linearly polarized light different from the one on an outward path by the quarter-wave plate 116. The red laser light reflected by the wedge-shaped mirror 115 is incident as P-polarized light on the polarization beam splitter 103 and the flat-plate beam splitter 113 after passing through the collimator lens 104. The red laser light having passed through the polarization beam splitter 103 and the flat-plate beam splitter 113 is introduced to the light receiving element 123 via the detection hologram 121 and the detection lens 122. The red laser light detected by the light receiving element 123 is photoelectrically converted. A signal generated by the photoelectric conversion is calculated by the unillustrated controller to generate a focus error signal used to follow a surface runout of the DVD 70 and a tracking error signal used to follow the eccentricity of the DVD 70.
Next, the operation of the optical head 140 in the case of recording or reproducing information on or from a CD 80 is described. Infrared laser light having a wavelength of about 785 nm and emitted from the two-wavelength laser light source 111 is separated into a main beam as 0th-order light and sub beams as ±1st-order diffracted light by the diffraction grating 112. The main beam and the sub beams are reflected by the flat-plate beam splitter 113 and pass through the polarization beam splitter 103. The infrared laser light having passed through the polarization beam splitter 103 is converted into substantially parallel light by the collimator lens 104 and incident on the wedge-shaped mirror 115. A part of the infrared laser light incident on the wedge-shaped mirror 115 is reflected toward the quarter-wave plate 116. The other part of the infrared laser light incident on the wedge-shaped mirror 115 is incident on the front monitor sensor 124 after passing through the wedge-shaped mirror 115 and the flat-plate mirror 105. The output of the infrared laser light of the two-wavelength laser light source 111 is controlled based on the output of the front monitor sensor 124.
On the other hand, the infrared laser light reflected by the wedge-shaped mirror 115 is focused as a light spot on an information recording surface of the CD 80 by the compatible objective lens 118 after passing through the quarter-wave plate 116.
The infrared laser light reflected by the information recording surface of the CD 80 is reflected by the wedge-shaped mirror 115 after passing through the compatible objective lens 118 and the quarter-wave plate 116 again. The infrared laser light reflected by the wedge-shaped mirror 115 passes through the polarization beam splitter 103 and the flat-plate beam splitter 113 after passing through the collimator lens 104. The infrared laser light having passed through the flat-plate beam splitter 113 is introduced to the light receiving element 123 via the detection hologram 121 and the detection lens 122. The infrared laser light detected by the light receiving element 123 is photoelectrically converted. A signal generated by the photoelectric conversion is calculated by the unillustrated controller to generate a focus error signal used to follow a surface runout of the CD 80 and a tracking error signal used to follow the eccentricity of the CD 80.
The optical head for recording or reproducing information on or from information recording media such as optical discs includes the front monitor sensor for detecting a part of laser light emitted from the light source in order to more accurately control the output of the laser light emitted from the light source particularly for recording. A detection signal in this front monitor sensor is an APC (Auto Power Control) signal. The APC signal is fed back to the controller for controlling the output of the light source. The APC signal is used to control the output of the light source so that suitable power necessary to record and/or reproduce information can be obtained.
Here, in the optical construction shown in FIG. 20, laser light emitted from the light source is converted into substantially parallel light, for example, by the collimator lens or the like and this substantially parallel light propagates toward the front monitor sensor by passing through or being reflected by the parallel-plate reflecting mirror. In such an optical construction, an optical axis of the laser light propagating toward the front monitor sensor after passing through or being reflected by the parallel-plate reflecting mirror and that of the laser light propagating toward the front monitor sensor after being internally reflected in the reflecting mirror become substantially parallel, thereby causing an interference. As a result, the APC signal in the front monitor sensor can be no longer accurately proportional to the output of the light source.
Accordingly, Japanese Unexamined Patent Publication No. 2004-5944 discloses such an optical construction that convergent or divergent light is incident on a parallel-plate beam splitter, so that the interference of laser light caused by internal reflection in the parallel-plate beam splitter is suppressed. Japanese Unexamined Patent Publication No. 2004-5944 also discloses an optical construction for suppressing the interference of laser light caused by internal reflection in a wedge-shaped beam splitter by using the wedge-shaped beam splitter.
For higher capacity of optical discs, it is thought to adopt a multi-layer structure comprised of three or more layers for information recording surfaces of high-density optical discs such as BDs. In an optical disc including a plurality of information recording surfaces, it is necessary to ensure a specified spacing between information recording surfaces in order to suppress the influence of reflected light (stray light) from the adjacent information recording surfaces (crosstalk of information signal, offset of a servo signal caused by stray light reflected by the adjacent information recording surfaces, etc.). Accordingly, in a multi-layer optical disc including three or more information recording surfaces, a spacing between an information recording surface having the largest light transmitting layer thickness and the one having the smallest light transmitting layer thickness has to be larger than in conventional two-layer discs.
Accordingly, at the time of recording or reproducing information on or from such a multi-layer optical disc, a created third-order spherical aberration increases in proportion to a deviation from an optimal light transmitting layer thickness of the objective lens. Thus, in an optical head for multi-layer optical discs, a movable range of a collimator lens needs to be larger than in conventional optical heads so as to be able to correct a larger third-order spherical aberration.
In the conventional optical head 140 shown in FIG. 20, the blue-violet laser light incident on the wedge-shaped mirror 115 is made non-parallel (divergent or convergent) by moving the collimator lens 104 in the optical axis direction in order to correct a third-order spherical aberration created according to the light transmitting layer thickness at the time of recording or reproducing information on or from the BD 90. Thus, the amount of third-order astigmatism of the blue-violet laser light having passed through the wedge-shaped mirror 115 changes.
FIG. 21 is a graph chart of a calculation result showing how the third-order astigmatism changed for each apex α of the wedge-shaped mirror 115 when the collimator lens was moved according to the light transmitting layer thickness. In FIG. 21, a horizontal axis represents the light transmitting layer thickness and a vertical axis represents the amount of third-order astigmatism. In FIG. 21, a graph 201 indicates a change of third-order astigmatism in relation to the light transmitting layer thickness when the apex α is +0.1°, a graph 202 indicates a change of third-order astigmatism in relation to the light transmitting layer thickness when the apex α is +0.06°, a graph 203 indicates a change of third-order astigmatism in relation to the light transmitting layer thickness when the apex α is 0°, a graph 204 indicates a change of third-order astigmatism in relation to the light transmitting layer thickness when the apex α is −0.06°, and a graph 205 indicates a change of third-order astigmatism in relation to the light transmitting layer thickness when the apex α is −0.1°. Calculation conditions were as follows.
Designed wavelength of the objective lens: 405 nmDesigned light transmitting layer thickness of87.5 μmthe objective lens:Focal length of the objective lens: 1.3 mmNumerical aperture (NA) of the objective lens:0.855Thickness of the wedge-shaped mirror: 1.0 mmRefractive index of the wedge-shaped mirror:1.53
It can be understood from FIG. 21 that the amount of third-order astigmatism created when the collimator lens was moved according to the light transmitting layer thickness changed depending on the apex α of the wedge-shaped mirror 115, through which the laser light passed, and was minimum when the apex α of the wedge-shaped mirror 115 was 0°, i.e. an incident surface and a reflecting surface were parallel.
Accordingly, if the wedge-shaped reflecting mirror is used to suppress the interference of the laser light caused by internal reflection as in the conventional optical head 140 of FIG. 20, the movable range of the collimator lens needs to be increased particularly for multi-layer optical discs including three or more information recording surfaces. Therefore, there is a problem that the change in the amount of third-order astigmatism considerably increases.
It is known that an amount of third-order coma aberration caused by disc tilt and that caused by the inclination of the objective lens (hereinafter, also referred to as lens tilt) respectively change according to the light transmitting layer thickness of the optical disc when the light transmitting layer thickness changes. The amount of third-order coma aberration created when the optical disc is inclined by a specified angle (at the time of disc tilt) increases in proportion to the light transmitting layer thickness. Further, the amount of third-order coma aberration created when the objective lens is inclined by a specified angle (at the time of lens tilt) decreases as the light transmitting layer thickness increases.
Thus, in the case of recording or reproducing information on or from an information recording surface having a large light transmitting layer thickness, the objective lens has to be largely inclined to correct a third-order coma aberration caused by the disc tilt. However, generally speaking, a third-order astigmatism is created according to the inclination of the objective lens if the objective lens is inclined.
FIG. 22 is a diagram showing the arrangement of the optical head in a conventional optical disc device. FIG. 23 is a diagram showing a state when the optical head accesses an inner circumferential side and an outer circumferential side of an optical disc in the conventional optical disc device.
In the general optical disc device, the optical head is arranged such that an optical axis of the collimator lens 104 and a tangential direction to an optical disc (DVD 70, CD 80 or BD 90) coincide as shown in FIG. 22. As shown in FIG. 22, laser light incident in the tangential direction of the optical disc is reflected in a direction perpendicular to an information recording surface of the optical disc by a wedge-shaped mirror 115 and a flat-plate mirror 105 and focused on the information recording surface of the optical disc by the objective lens 108 or the compatible objective lens 118. By arranging in this way, an access to the innermost circumferential side of the optical disc becomes easier and a projecting mount of the optical head from the outer circumferential side of the optical disc when the optical head accesses the outermost circumferential side of the optical disc becomes smaller as shown in FIG. 23.
However, if the optical head is arranged such that the optical axis of the collimator lens and the tangential direction of the optical disc coincide, a first third-order astigmatism created when the collimator lens is moved in the optical axis direction to correct a third-order spherical aberration and a second third-order astigmatism created when the objective lens is inclined in the radial direction of the optical disc to correct a third-order coma aberration have the same direction component (0°/90° direction) and the same polarity.
As described above, both first third-order astigmatism and second third-order astigmatism increase at the time of recording or reproducing information on or from the information recording surface having a large light transmitting layer thickness. Thus, particularly in the optical head for multi-layer optical discs including three or more information recording surfaces, the recording or reproduction of information may be largely influenced by the addition of the first and second third-order astigmatisms.